1) Guardians are kind of messed up. Often, when they parry an attack (which they're supposed to as a defensive specialist) it delays their next action.
2) The key to being a Jedi Knight is split-second timing, which is difficult or impossible if you are one of the people who suffer from ability delay.
3) As a tank, Guardians are just poorly constructed. They are hard to play, yet do little damage and don't have a tank's survivability.